1. Field of the Invention
The present invention relates to a method of controlling positions and vibrations and an active vibration control apparatus therefor for carrying out a precise vibration control for any kind of precision machines and precision instruments such as a holographic instrument, an electron microscope, apparatus for producing semiconductors, a laser measuring machine, a super precision measuring machine and so sorth.
2. Description of the Prior Art
A floor receives any kind of vibrations such as strong and weak vibrations, horizontal and vertical vibrations and any range of frequencies of vibrations from high frequency to low frequency. Therefore, these vibrations are propagated to a precision machine or instrument which is mounted on the floor.
For instance where a measuring instrument mounted on the floor provides driving means and when a work piece to be measured by the instrument is moved, vibrations occur and inclination of the instrument may sometimes be occurred.
In the case that the instrument or the machine is an ordinary machine tool, it is not necessary to consider influence of such vibrations propagated to the machine. However, in the case that the instrument or machine is such as a jig boring machine, a super precision machine tool an apparatus for producing a super LSI, or a laser measuring machine, such vibrations give a fatal influence to those instruments.
Accordingly, vibration control becomes a very important element in keeping a high recision for the super precision machine. Since the vibrations propagated to the super precision machine on the floor involve many kinds of frequencies and sometimes the machine resonates according to a certain frequency, and an ordinary spring cannot control such vibrations.
Therefore, instead of the conventional idea of the vibration control system, it is introduced an active vibration control system which converts the vibrations propagated from the floor into electronic signals so as to control a vibration control base by means of an active vibration control apparatus.
For example, Japanese Patent Application No. 63-35829, discloses a vibration control apparatus for controlling vibrations and maintaining a level of a vibration control base by means of a digital control system, which is shown in FIG. 28.
The apparatus provides a digital level control circuit 100 for controlling a level. The digital control circuit detects a value of level displacement of a vibration control base 131 by means of a level sensor 120 and the output of the level sensor 120 is compared with a standard level voltage by means of a comparator 132 and the difference obtained thereby is input to a pulse generator 121 so that the generator generates analog signals in accordance with the value of the displacement and then inputs the analog signals into a displacement pulse generator 121. Constant pulses are generated by the displacement pulse generator 121 in synchronism with a clock 122, which is provided therewith, during the period of generation of the analog signals and the constant pulses are input to an adder 123. The adder 123 counts the input signals and then outputs same as the displacement signals and thereby the value of level displacement may be sized digitally.
Next, vertical micro-vibrations may be controlled by means of a digital vibration control circuit 101. Micro-vibrations of a vibration control base 131 propagated from a floor or vibrations of a precision instrument on the base, are detected by a vibration sensor 124 and vibration signals from the vibration sensor 124 are input to an arithmetic circuit 125 via a lowpass filter 133. The phases of the detected signals output from the arithmetic circuit 125 are inverted at 180.degree. by means of a phase inverter 126. The signals inverted by the phase inverter 126 are input to a level displacement and micro-vibration adder 127 after they are changed to analog signals. By means of the level displacement and micro-vibration adder 127, the phase inverting signals are added to the level displacement signals output from the adder 123 of the level control circuit 100 and disturbance control is carried out together with a levelling correction. The outputs from the level displacement and micro-vibration adder 127 are input to a drive circuit 128 so as to control a control valve 129 in order to control a pressure of an air spring 132. An compression air is supplied to the control valve 129 from an air pressure source 110 via a regulator 112 so as to control an air pressure of the air spring 132.
According to the digital level control circuit 100, the drive circuit 128 outputs a raising signal to the control valve 129 in order to lift up the vibration control base 131 by means of the air spring 132 when the vibration control base 131 goes down below a standard level H, and when the vibration control base 131 becomes positioned above the standard level H, a lowering signal is output.
While the raising signal is output, the output signals of the displacement pulse generator 121 are added by means of the adder 123 and subtracted thereby when the lowering signal is output. The added or subtracted signals are finally converted to analog signals via a D/A converter 130 and then output. The addition and subtraction is continued while the vibration control base 131 is not accorded with the standard level H.
Such a digital control system cannot, however, evade hunting (raising and lowering) of the vibration control base 131 around the standard level H since the raising and lowering signals are changed on the basis of a single point of the standard level. Therefore, it is required to add another means to the digital control system so as to stop counting of the adder 123 within a range which is limited at very short distances in up and down directions from the standard level. However, in such a manner, the vibration control base 131 is freely raised and lowered within the limited range so that a level error frequently occurs within the limited range. In other words, the digital control system includes a control precision of .+-.X % at the standard level H.
To minimize the error and increase the precision of levelling, it is necessary to use a D/A converter 130 having a great number of bits. However, such a converter increases a cost of the apparatus.